M R I m a g i n g of M e d i a s t i n a l Ma s s e s Jeanne B. Ackman, MD KEYWORDS MR imaging Mediastinal mass Tissue characterization Cystic Fibrous Hemorrhagic Diagnostic specificity Clinical management
KEY POINTS The high soft tissue contrast of MR imaging enables superior tissue characterization of mediastinal masses, compared with computed tomography, and often increases diagnostic specificity. Appropriate use of mediastinal MR imaging may prevent unnecessary diagnostic intervention. When intervention is needed, mediastinal MR imaging can direct interventionists toward the optimal site for biopsy or the correct compartment for resection.
The high soft tissue contrast of MR imaging, relative to other imaging modalities including computed tomography (CT), enables superior tissue characterization of many lesions throughout the body, including those in the mediastinum. The result, in many cases, is added diagnostic specificity or virtual biopsy of the lesion. Much has been written about mediastinal masses and how their differential diagnosis can be narrowed by determination of their mediastinal compartment of origin. The premise of this method is that knowledge of the structures that normally reside in a given mediastinal compartment reduces the number of diagnostic possibilities. This article describes how adding MR imaging to the diagnostic armamentarium yields further diagnostic precision, with potential to improve clinical management. Therefore, this article is organized not by mediastinal compartment but by MR findings that distinguish one mediastinal mass tissue type from another. The following categories are discussed in the context of their diagnostic significance, as they particularly highlight the value of MR imaging in the mediastinum:
Discernment of cystic from solid lesions Corollary: discernment of solid tissue amidst hemorrhage and necrosis; guidance for diagnostic intervention Detection of macroscopic and microscopic fat Detection of lesion T2-hypointensity Demonstration of a lesion’s dynamic contrast enhancement pattern Demonstration of matching mediastinal lesions in terms of signal and enhancement in the same patient and its significance Detection of low apparent diffusion coefficient (ADC) values Determination of lesion invasiveness Discernment of mediastinal from paramediastinal lesions The new International Thymic Malignancy Interest Group (ITMIG) classification of mediastinal compartments,1 developed by a consensus of its members, is used when describing the compartment of origin of the mass under discussion in the article. Instead of basing lesion location on mediastinal compartments delineated by lines drawn on a lateral chest radiograph,2 this more
Disclosure: The author has nothing to disclose. Division of Thoracic Imaging and Intervention, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Founders 202, 55 Fruit Street, Boston, MA 02114, USA E-mail address: [email protected] Magn Reson Imaging Clin N Am 23 (2015) 141–164 http://dx.doi.org/10.1016/j.mric.2015.01.002 1064-9689/15/$ – see front matter Ó 2015 Elsevier Inc. All rights reserved.
mri.theclinics.com
INTRODUCTION
Ackman
142
modern classification bases lesion location on its relationship to 3 compartments delineated by cross-sectional imaging that extend from the thoracic inlet to the diaphragm: a prevascular (anterior mediastinal) compartment, including all structures anterior to the pericardium and proximal ascending aorta; a visceral (middle mediastinal) compartment, including all major mediastinal visceral structures extending from anterior pericardium posteriorly to a vertical line drawn 1 cm posterior to the anterior margin of the spine (both the trachea and the esophagus are therefore included in this middle mediastinal compartment); and a paravertebral (posterior mediastinal) compartment, including all mediastinal structures
posterior to this vertical line. A list of mediastinal masses typically found in each of these compartments is provided in Table 1.
MEDIASTINAL MAGNETIC RESONANCE PROTOCOL High quality mediastinal MR protocols involve pulse sequences required to adequately characterize a lesion. These protocols generally include T1-weighted, T2-weighted, and T2-weighted fat-saturated pulse sequences, as well as pre-gadolinium and post-gadolinium threedimensional (3D) ultrafast gradient echo (GRE) dynamic contrast-enhanced imaging. Ultrafast
Table 1 Mediastinal masses by compartmenta Anterior Mediastinum or Prevascular Compartmenta
Middle Mediastinum or Visceral Compartment
Posterior Mediastinum or Paravertebral Compartment
Lymphadenopathy Thyroid and parathyroid lesions —
Lymphadenopathy Thyroid lesions Ascending aortic aneurysm Aortic arch aneurysm Dilated main pulmonary artery Aberrant right/left subclavian artery Descending aortic aneurysm —
Lymphadenopathy — Descending aortic aneurysm
Paraganglioma and other neurogenic tumors — — Pancreatic pseudocyst Mesothelial cysts
Neurogenic tumors including neurofibroma, schwannoma, paraganglioma Lateral meningocele — Pancreatic pseudocyst Mesothelial cysts
— Tracheal lesions Esophageal lesions Hiatal hernia Foregut duplication cysts Abscess Hematoma Fibrosing mediastinitis Hemangioma Lymphangioma Sarcoma
Extramedullary hematopoiesis — — Bochdalek hernia Foregut duplication cysts Abscess Hematoma Fibrosing mediastinitis Hemangioma Lymphangioma Sarcoma
Thymic lesions (cyst, hyperplasia, thymic neoplasm, including lymphoma) —
— Germ cell tumors — Pleuropericardial or mesothelialb cysts — — — Morgagni hernia Bronchogenic cysts (very rarely) Abscess Hematoma Fibrosing mediastinitis Hemangioma Lymphangioma Sarcoma a b
—
Mediastinal compartments, as prescribed by the new ITMIG classification. Mesothelial cysts may be found anywhere in the body where mesothelium exists.
MR Imaging of Mediastinal Masses GRE in-phase and out-of-phase chemical shift MR imaging is recommended for T1-weighted sequences because lesion coverage for both phases can be acquired in a single 20-second breath hold and additional information is obtained regarding the presence or absence of microscopic or intravoxel fat within a lesion at no additional time cost. Supplemental diffusion-weighted and short tau inversion recovery imaging can be performed if the finding of low ADC values is anticipated to refine the differential diagnosis or if the detection of bone marrow edema or involvement is diagnostically or therapeutically critical, respectively. Breath-hold imaging for all pulse sequences, including the 3-plane localizer, is strongly preferred to respiratory gating because it more reliably freezes respiratory motion and dispenses with associated respiratory motion artifact. Breath-hold imaging is almost universally successful in patients, from adolescent to elderly, provided the technologist rehearses breath-holding with the patient before the patient lies down on the table, primes the patient before each pulse sequence about the nature (length, frequency) of the breath holds, and provides MR-compatible 2-L nasal cannula oxygen3 when breath-hold difficulty is anticipated. If a patient requires higher amounts of oxygen therapy at home or in the hospital, the volume of oxygen delivery should be suitably adjusted. When cardiac gating is used, electrocardiogram (ECG) gating is strongly preferred to peripheral gating because ECG gating more reliably freezes cardiac motion and virtually eradicates associated pulsatility artifact.4 A suggested mediastinal MR imaging protocol is provided in Table 2. For noninvasive thymic lesion evaluation, a shortened version of this protocol can be used (Box 1).
TISSUE CHARACTERIZATION OF MEDIASTINAL MASSES: ITS DIAGNOSTIC AND THERAPEUTIC SIGNIFICANCE The Diagnostic Significance of Discernment of Cystic from Solid Lesions It is easy to misinterpret cystic lesions as solid on both noncontrast and contrast-enhanced CT.5 Many cystic lesions in the mediastinum contain hemorrhage, proteinaceous material, and occasionally calcium oxalate (milk of calcium), all of which may increase the attenuation of a lesion and cause the lesion to appear solid on CT. Although recalling the patient for a subsequent CT scan without and with intravenous (IV) contrast could help demonstrate whether a lesion enhances, it is not practical because it substantially increases
radiation exposure and may not detect more subtly enhancing septations or small mural nodules. Dualsource CT offers some promise in this context, but it too may not show subtle lesion complexity with the same efficacy as MR imaging on account of its inferior soft tissue contrast to MR imaging, and it can not do so without radiation exposure. Thymic cysts are instructive in this regard. Research has shown that these benign lesions are often misinterpreted as thymomas on CT.5 Thymic cysts may be unilocular or multilocular and may be solitary or associated with thymic hyperplasia, thymic neoplasms, and lymphoma. Congenital thymic cysts are typically unilocular and acquired thymic cysts may be unilocular or multilocular. These acquired thymic cysts, unlike their congenital counterparts, do not usually contain thymic epithelium in their walls and generally arise in the setting of inflammation and fibrosis, whether from prior surgery, trauma, infectious/inflammatory processes (human immunodeficiency virus [HIV], autoimmune), chemotherapy, or radiation. They arise from the thymopharyngeal duct remnant and may therefore occur anywhere along a line from the angle of the mandible to the thymic bed.6,7 These lesions may be of variable attenuation on CT and of variable T1 signal on MR imaging.8 The CT appearance of these cystic lesions can be misleading and may even prompt inappropriate intervention in some cases (Figs. 1–3).5 Unilocular thymic cysts can be misinterpreted as thymomas, as was the case with the example in Fig. 1. It is critical to be aware that thymic cysts have been observed to measure up to 97 Hounsfield units (HU)5 and that they can fluctuate both in size and attenuation over time and still be benign (see Fig. 2). A simple cyst in this context refers to a well-circumscribed, fluid-containing lesion, with a barely perceptible or thin, smooth wall and no internal lesion complexity, whether or not it contains hemorrhagic or proteinaceous material and whether or not the wall enhances. Two examples of multilocular thymic cysts are provided in Fig. 3. An important caveat to the interpretation of a simple thymic cyst by MR imaging is that the malignant potential of simple cysts in the thymic bed found by MR imaging is not known. The risk of cystic thymoma arising from one of these simple cysts is apt to be low given the rarity of these tumors and the extremely low malignant potential of simple cysts found elsewhere in the body by MR imaging. Nevertheless, until future investigation reveals otherwise, these lesions warrant follow-up. The consensus reached between thoracic radiologists and thoracic surgeons at our quaternary referral institution is that these cysts be followed for up to 5 years, should they
143
144
Table 2 Mediastinal MR imaging protocol
Pulse Sequence
GE
Siemens
Philips
TR (ms)a
TE (ms)b
BH axial and sagittal SSFP balanced gradient echo (GRE)
FIESTA
True FISP
BFFE
3–270
BH coronal ultrafast spin echo T2
SSFSE
HASTE
UFSE
1.2–1.5 (minimum full) 80–100
BH sagittal UFSE fat-saturated T2
SSFSE
HASTE
UFSE
BH axial in-phase and out-of-phase (chemical shift) ultrafast GRE T1 (dual echo preferable) BH cardiac gated double IR T2 BH before and after 3D ultrafast GRE with automated subtraction (postgadolinium imaging) acquired at 20 s (axial), 1 min (axial), 3 min (sagittal), and 5 min (axial)c upon administration of 20cc of IV gadolinium Optional coronal/sagittal BH STIR Optional diffusion-weighted echo planar imaging Optional BH cardiac gated double inversion recovery T1 Optional BH cardiac gated sagittal double inversion recovery fat-saturated T2 Optional respiratory-triggered axialdriven equilibrium without and with fat saturation
FSPGR
TurboFLASH
TFE
900–1000 NA 1 (minimum) 900–1000 80–100 NA 1 (minimum) 115–190 4.2–4.8/2.1–2.4 70–80 1
45–80 1
NA 1 10–12
KEY POINTS The high soft tissue contrast of MR imaging enables superior tissue characterization of mediastinal masses, compared with computed tomography, and often increases diagnostic specificity. Appropriate use of mediastinal MR imaging may prevent unnecessary diagnostic intervention. When intervention is needed, mediastinal MR imaging can direct interventionists toward the optimal site for biopsy or the correct compartment for resection.
The high soft tissue contrast of MR imaging, relative to other imaging modalities including computed tomography (CT), enables superior tissue characterization of many lesions throughout the body, including those in the mediastinum. The result, in many cases, is added diagnostic specificity or virtual biopsy of the lesion. Much has been written about mediastinal masses and how their differential diagnosis can be narrowed by determination of their mediastinal compartment of origin. The premise of this method is that knowledge of the structures that normally reside in a given mediastinal compartment reduces the number of diagnostic possibilities. This article describes how adding MR imaging to the diagnostic armamentarium yields further diagnostic precision, with potential to improve clinical management. Therefore, this article is organized not by mediastinal compartment but by MR findings that distinguish one mediastinal mass tissue type from another. The following categories are discussed in the context of their diagnostic significance, as they particularly highlight the value of MR imaging in the mediastinum:
Discernment of cystic from solid lesions Corollary: discernment of solid tissue amidst hemorrhage and necrosis; guidance for diagnostic intervention Detection of macroscopic and microscopic fat Detection of lesion T2-hypointensity Demonstration of a lesion’s dynamic contrast enhancement pattern Demonstration of matching mediastinal lesions in terms of signal and enhancement in the same patient and its significance Detection of low apparent diffusion coefficient (ADC) values Determination of lesion invasiveness Discernment of mediastinal from paramediastinal lesions The new International Thymic Malignancy Interest Group (ITMIG) classification of mediastinal compartments,1 developed by a consensus of its members, is used when describing the compartment of origin of the mass under discussion in the article. Instead of basing lesion location on mediastinal compartments delineated by lines drawn on a lateral chest radiograph,2 this more
Disclosure: The author has nothing to disclose. Division of Thoracic Imaging and Intervention, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Founders 202, 55 Fruit Street, Boston, MA 02114, USA E-mail address: [email protected] Magn Reson Imaging Clin N Am 23 (2015) 141–164 http://dx.doi.org/10.1016/j.mric.2015.01.002 1064-9689/15/$ – see front matter Ó 2015 Elsevier Inc. All rights reserved.
mri.theclinics.com
INTRODUCTION
Ackman
142
modern classification bases lesion location on its relationship to 3 compartments delineated by cross-sectional imaging that extend from the thoracic inlet to the diaphragm: a prevascular (anterior mediastinal) compartment, including all structures anterior to the pericardium and proximal ascending aorta; a visceral (middle mediastinal) compartment, including all major mediastinal visceral structures extending from anterior pericardium posteriorly to a vertical line drawn 1 cm posterior to the anterior margin of the spine (both the trachea and the esophagus are therefore included in this middle mediastinal compartment); and a paravertebral (posterior mediastinal) compartment, including all mediastinal structures
posterior to this vertical line. A list of mediastinal masses typically found in each of these compartments is provided in Table 1.
MEDIASTINAL MAGNETIC RESONANCE PROTOCOL High quality mediastinal MR protocols involve pulse sequences required to adequately characterize a lesion. These protocols generally include T1-weighted, T2-weighted, and T2-weighted fat-saturated pulse sequences, as well as pre-gadolinium and post-gadolinium threedimensional (3D) ultrafast gradient echo (GRE) dynamic contrast-enhanced imaging. Ultrafast
Table 1 Mediastinal masses by compartmenta Anterior Mediastinum or Prevascular Compartmenta
Middle Mediastinum or Visceral Compartment
Posterior Mediastinum or Paravertebral Compartment
Lymphadenopathy Thyroid and parathyroid lesions —
Lymphadenopathy Thyroid lesions Ascending aortic aneurysm Aortic arch aneurysm Dilated main pulmonary artery Aberrant right/left subclavian artery Descending aortic aneurysm —
Lymphadenopathy — Descending aortic aneurysm
Paraganglioma and other neurogenic tumors — — Pancreatic pseudocyst Mesothelial cysts
Neurogenic tumors including neurofibroma, schwannoma, paraganglioma Lateral meningocele — Pancreatic pseudocyst Mesothelial cysts
— Tracheal lesions Esophageal lesions Hiatal hernia Foregut duplication cysts Abscess Hematoma Fibrosing mediastinitis Hemangioma Lymphangioma Sarcoma
Extramedullary hematopoiesis — — Bochdalek hernia Foregut duplication cysts Abscess Hematoma Fibrosing mediastinitis Hemangioma Lymphangioma Sarcoma
Thymic lesions (cyst, hyperplasia, thymic neoplasm, including lymphoma) —
— Germ cell tumors — Pleuropericardial or mesothelialb cysts — — — Morgagni hernia Bronchogenic cysts (very rarely) Abscess Hematoma Fibrosing mediastinitis Hemangioma Lymphangioma Sarcoma a b
—
Mediastinal compartments, as prescribed by the new ITMIG classification. Mesothelial cysts may be found anywhere in the body where mesothelium exists.
MR Imaging of Mediastinal Masses GRE in-phase and out-of-phase chemical shift MR imaging is recommended for T1-weighted sequences because lesion coverage for both phases can be acquired in a single 20-second breath hold and additional information is obtained regarding the presence or absence of microscopic or intravoxel fat within a lesion at no additional time cost. Supplemental diffusion-weighted and short tau inversion recovery imaging can be performed if the finding of low ADC values is anticipated to refine the differential diagnosis or if the detection of bone marrow edema or involvement is diagnostically or therapeutically critical, respectively. Breath-hold imaging for all pulse sequences, including the 3-plane localizer, is strongly preferred to respiratory gating because it more reliably freezes respiratory motion and dispenses with associated respiratory motion artifact. Breath-hold imaging is almost universally successful in patients, from adolescent to elderly, provided the technologist rehearses breath-holding with the patient before the patient lies down on the table, primes the patient before each pulse sequence about the nature (length, frequency) of the breath holds, and provides MR-compatible 2-L nasal cannula oxygen3 when breath-hold difficulty is anticipated. If a patient requires higher amounts of oxygen therapy at home or in the hospital, the volume of oxygen delivery should be suitably adjusted. When cardiac gating is used, electrocardiogram (ECG) gating is strongly preferred to peripheral gating because ECG gating more reliably freezes cardiac motion and virtually eradicates associated pulsatility artifact.4 A suggested mediastinal MR imaging protocol is provided in Table 2. For noninvasive thymic lesion evaluation, a shortened version of this protocol can be used (Box 1).
TISSUE CHARACTERIZATION OF MEDIASTINAL MASSES: ITS DIAGNOSTIC AND THERAPEUTIC SIGNIFICANCE The Diagnostic Significance of Discernment of Cystic from Solid Lesions It is easy to misinterpret cystic lesions as solid on both noncontrast and contrast-enhanced CT.5 Many cystic lesions in the mediastinum contain hemorrhage, proteinaceous material, and occasionally calcium oxalate (milk of calcium), all of which may increase the attenuation of a lesion and cause the lesion to appear solid on CT. Although recalling the patient for a subsequent CT scan without and with intravenous (IV) contrast could help demonstrate whether a lesion enhances, it is not practical because it substantially increases
radiation exposure and may not detect more subtly enhancing septations or small mural nodules. Dualsource CT offers some promise in this context, but it too may not show subtle lesion complexity with the same efficacy as MR imaging on account of its inferior soft tissue contrast to MR imaging, and it can not do so without radiation exposure. Thymic cysts are instructive in this regard. Research has shown that these benign lesions are often misinterpreted as thymomas on CT.5 Thymic cysts may be unilocular or multilocular and may be solitary or associated with thymic hyperplasia, thymic neoplasms, and lymphoma. Congenital thymic cysts are typically unilocular and acquired thymic cysts may be unilocular or multilocular. These acquired thymic cysts, unlike their congenital counterparts, do not usually contain thymic epithelium in their walls and generally arise in the setting of inflammation and fibrosis, whether from prior surgery, trauma, infectious/inflammatory processes (human immunodeficiency virus [HIV], autoimmune), chemotherapy, or radiation. They arise from the thymopharyngeal duct remnant and may therefore occur anywhere along a line from the angle of the mandible to the thymic bed.6,7 These lesions may be of variable attenuation on CT and of variable T1 signal on MR imaging.8 The CT appearance of these cystic lesions can be misleading and may even prompt inappropriate intervention in some cases (Figs. 1–3).5 Unilocular thymic cysts can be misinterpreted as thymomas, as was the case with the example in Fig. 1. It is critical to be aware that thymic cysts have been observed to measure up to 97 Hounsfield units (HU)5 and that they can fluctuate both in size and attenuation over time and still be benign (see Fig. 2). A simple cyst in this context refers to a well-circumscribed, fluid-containing lesion, with a barely perceptible or thin, smooth wall and no internal lesion complexity, whether or not it contains hemorrhagic or proteinaceous material and whether or not the wall enhances. Two examples of multilocular thymic cysts are provided in Fig. 3. An important caveat to the interpretation of a simple thymic cyst by MR imaging is that the malignant potential of simple cysts in the thymic bed found by MR imaging is not known. The risk of cystic thymoma arising from one of these simple cysts is apt to be low given the rarity of these tumors and the extremely low malignant potential of simple cysts found elsewhere in the body by MR imaging. Nevertheless, until future investigation reveals otherwise, these lesions warrant follow-up. The consensus reached between thoracic radiologists and thoracic surgeons at our quaternary referral institution is that these cysts be followed for up to 5 years, should they
143
144
Table 2 Mediastinal MR imaging protocol
Pulse Sequence
GE
Siemens
Philips
TR (ms)a
TE (ms)b
BH axial and sagittal SSFP balanced gradient echo (GRE)
FIESTA
True FISP
BFFE
3–270
BH coronal ultrafast spin echo T2
SSFSE
HASTE
UFSE
1.2–1.5 (minimum full) 80–100
BH sagittal UFSE fat-saturated T2
SSFSE
HASTE
UFSE
BH axial in-phase and out-of-phase (chemical shift) ultrafast GRE T1 (dual echo preferable) BH cardiac gated double IR T2 BH before and after 3D ultrafast GRE with automated subtraction (postgadolinium imaging) acquired at 20 s (axial), 1 min (axial), 3 min (sagittal), and 5 min (axial)c upon administration of 20cc of IV gadolinium Optional coronal/sagittal BH STIR Optional diffusion-weighted echo planar imaging Optional BH cardiac gated double inversion recovery T1 Optional BH cardiac gated sagittal double inversion recovery fat-saturated T2 Optional respiratory-triggered axialdriven equilibrium without and with fat saturation
FSPGR
TurboFLASH
TFE
900–1000 NA 1 (minimum) 900–1000 80–100 NA 1 (minimum) 115–190 4.2–4.8/2.1–2.4 70–80 1
45–80 1
NA 1 10–12
